Method and apparatus for performing digital pre-distortion

ABSTRACT

A pre-distorter that compensates for amplitude and phase distortion created by an amplifier. During a training session, the amplifier is stimulated with input signals of pre-selected amplitude and phase at various temperatures and the amplifier output is captured and converted into data sets. Polynomials are then fitted to the data sets and inverses of the polynomials are determined. The coefficients of the inverse polynomials are then saved for each temperature. During operation, the amplifier temperature is predicted based on the amplifier input signal and the coefficients associated with the predicted temperature are selected to be applied to the input signal to compensate for amplitude and phase distortion caused by the amplifier.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of Ser. No. 10/919,029,filed on Aug. 16, 2004, which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to compensating fornon-linearities in amplifiers and, more particularly, to a method and anapparatus for performing digital pre-distortion in an amplifier tocompensate for the non-linearities in the amplifier.

BACKGROUND OF THE INVENTION

A typical power amplifier does not behave linearly. There are occasionswhen the power amplifier gives compression to the output of theamplifier and there are occasions when it gives expansion to the output.Typical signal detectors that receive and decode these amplified signalscannot operate in such a non-linear fashion. Therefore, it is necessaryto linearize the amplifier output by applying inverse distortion at theinput to the power amplifier to undo the compression or expansionproduced by the amplifier. Digital pre-distorters are commonly used withpower amplifiers to invert the power amplifier saturationcharacteristics by expanding the saturation regions and compressing theexpansion regions in the power amplifier characteristics curve.

FIG. 1 is a block diagram of a typical digital pre-distorter 1 and poweramplifier 2. The input signal received by the pre-distorter 1, V_(IN),is pre-distorted by the pre-distorter 1 into a pre-distorted signal,V_(PD). The pre-distorted signal V_(PD) is such that the non-linearitiesof the power amplifier 2 cause the power amplifier 2 to produce anoutput signal, V_(OUT) _(—) _(TARGET), that is closer to what an idealoutput signal of the power amplifier 2 should be.

The basic principles of a typical digital pre-distorter can be seen fromthe graph shown in FIG. 2. In FIG. 2, the vertical axis represents thevoltage output of a typical power amplifier and the horizontal axisrepresents the voltage input to the power amplifier. The linearoperation of the power amplifier is represented by the followingequation:y=mx+c,where m is the linear gain of the power amplifier and c=0 is the outputintercept point. This equation corresponds to line 3 in FIG. 2, whichhas a slope equal to m. The curve 4 in the graph of FIG. 2 correspondsto a non-pre-distorted output characteristic curve for a typical poweramplifier. The non-linearities of the power amplifier result inamplitude-to-amplitude (AM/AM) distortion as well as amplitude-to-phase(AM/PM) distortion, which results in curve 4 being non-linear.

A digital pre-distorter actually increases or decreases the amplifierinput magnitude to linearize the amplifier output, i.e., to make curve 4look more like line 3. The digital pre-distorter asserts a negativephase distortion to mitigate the phase distortion introduced by thepower amplifier. It can be seen from FIG. 2 that in order to increasethe amplifier output magnitude by ΔV_(OUT) so thatVout_target=Vout_nopd+ ΔVout, the amplifier input magnitude needs to beincreased by ΔV_(IN) so that Vpd=Vin+Δ Vin. The pre-distorter performsthese functions by first fitting an odd ordered polynomial to the poweramplifier magnitude characteristic curve (e.g., curve 4 in FIG. 2),calculating the inverse of the polynomial, and then applying thecoefficients of the inverse polynomial to the amplifier input in thepre-distorter to linearize the amplifier output. In essence, applyingthe coefficients to the amplifier input creates inverse distortion thatreverses any compression or expansion caused by the amplifier.

Furthermore, the amplifier output characteristics change over short andlong periods of time giving rise to what are commonly referred to asslow memory effect and fast memory effect. Therefore, the appropriatepolynomial must be selected for the appropriate circumstances. Then, itsinverse polynomial obtained, and then the coefficients of the inversepolynomial applied to the amplifier input to cause the amplifier outputcharacteristic curve to be altered. In addition, the selection of theappropriate polynomial and the application of the coefficients of itsinverse to the amplifier output characteristic curve should be done veryquickly, or as close to real-time as possible.

Slow memory effect is defined as changes to the amplifier outputcharacteristics due to aging, slow changes in ambient temperature,humidity, etc. Fast memory effect is defined as changes to the amplifieroutput characteristics due to instantaneous changes in the operatingtemperature of the amplifier. Because of these changes, it is generallynot sufficient to use the same inverse polynomial coefficients all ofthe time. FIGS. 3A and 3B are graphs illustrating the amplifier outputcharacteristic curves for various temperatures for amplitude and phase,respectively. The line 11 in FIG. 3 represents the target gain, whichcorresponds to the ideal amplifier output characteristic for amplitude.Curves 12-16 in FIG. 3 are the amplifier amplitude output characteristiccurves for operating temperatures T1-T5, respectively, whereT5<T4<T3<T2<T1. The amplifier curves 12-16 as a function of temperatureare commonly referred to as memory characteristic curves.

It is known to use adaptive negative feedback systems that measure theamplifier output and determine which polynomial coefficients to selectbased on the output of the amplifier. In such systems, based on themeasured amplifier output, it is determined which amplifier memorycharacteristic curve corresponds to the current temperature, and thecorresponding coefficients are applied to correct for gain and phase(delta coefficients for gain and distortion coefficients for phase). Onedisadvantage of these systems is that they typically use large arrays ofread-only memory (ROM) or complex lookup tables to store thecoefficients and complex computer processing, which increases powerconsumption and adds delay in the feedback loop. Also, such systemsoften use expensive temperature sensors to sense the amplifiertemperature to determine which coefficients to select. Sensing amplifiertemperature not only increases system costs, but also makes the systemprone to error. Furthermore, such systems are incapable of performingfast memory compensation using coefficients that correspond to anappropriate memory characteristic curve. Rather, a composite curve istypically used, which blends all temperatures into a resultant curve.Using the resultant curve rather than individual memory curves alsomakes the system more error prone.

Accordingly, a need exists for an amplifier digital pre-distorter thatdoes not require large memory arrays for storing the delta anddistortion coefficients, that does not require temperature sensors, andthat is capable of performing fast memory compensation without the needto use a resultant memory characteristic curve.

SUMMARY OF THE INVENTION

The present invention provides a pre-distorter apparatus, method andcomputer program for compensating for non-linearities in an amplifier.The pre-distorter comprises predictor circuitry and processingcircuitry. The predictor circuitry predicts the temperature of theamplifier based on an input signal and generates a predictor outputsignal that is based on the temperature prediction. The processingcircuitry selects inverse polynomial coefficients based on the predictoroutput signal and applies the coefficients to the input signal tocompensate for non-linearities in the amplifier.

The method of pre-distorting an amplifier input signal to compensate fornon-linearities of the amplifier comprises predicting a temperature ofthe amplifier based on the input signal and applying inverse polynomialcoefficients to the input signal that are selected based on theprediction. The coefficients applied are those that are associated withan output characteristic curve of the amplifier for the predictedtemperature.

The computer program of the present invention corresponds to a trainingalgorithm for calculating inverse polynomial coefficients that are to beapplied to an amplifier input signal to pre-distort the amplifier inputsignal to compensate for non-linearities of the amplifier. A firstroutine of the program captures a plurality of amplifier output signalsproduced by the amplifier in response to the amplifier being stimulatedby a plurality of respective amplifier input signals. The output signalscorrespond to respective amplifier temperatures. A second routine fitsrespective polynomials to respective data sets that represent respectiveamplifier output signals. A third routine determines respective inversesof the respective polynomials. A fourth routine obtains respective setsof inverse polynomial coefficients from the respective inverses of thepolynomials. The respective sets of coefficients correspond torespective temperatures of the amplifier.

These and other features and advantages of the present invention willbecome apparent from the following description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a typical digital pre-distorter and poweramplifier.

FIG. 2 is a graph demonstrating the basic principles of a typicaldigital pre-distorter.

FIG. 3 is a graph illustrating the amplifier output amplitudecharacteristic curve for various temperatures.

FIG. 4 is a block diagram of the pre-distorter of the present inventionand predictor circuitry in accordance with an embodiment.

FIG. 5 is a block diagram of the pre-distorter of the present inventionand predictor circuitry in accordance with another embodiment.

FIG. 6 is a block diagram of the digital signal processor (DSP),selection circuitry and coefficient injection circuitry of thepre-distorter of the present invention in accordance with an embodiment.

FIG. 7 is a flow chart of the pre-distortion method of the invention inaccordance with an embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, it has been determined that itis possible to predict which of the memory characteristic curves 12-16shown in FIG. 3 will correspond to the amplifier output characteristiccurve based on the energy of the amplifier input. Once the correctmemory characteristic curve has been selected, the correspondingcoefficients are applied to the amplifier input signal. Because theselection is based on the amplifier input rather than the amplifieroutput or some other measurement obtained by probing the amplifier, itis possible to select and apply the corresponding coefficients tocompensate fast memory effects of amplitude and phase distortion.

During a training session, the amplifier is stimulated with inputsignals of pre-selected amplitude and phase at various temperatures andthe amplifier output is captured and converted into data sets.Polynomials are then fitted to the data sets and inverses of thepolynomials are determined. The coefficients of the inverse polynomialsare then saved for each temperature. During operation, the amplifiertemperature is predicted based on the amplifier input signal and thecoefficients associated with the predicted temperature are selected tobe applied to the input signal to compensate for amplitude and phasedistortion caused by the amplifier. The manner in which these tasks areperformed will now be described with reference to FIGS. 4-7.

FIG. 4 is a block diagram of the present invention in accordance with anembodiment. In accordance with the embodiment represented by FIG. 4,predictor circuitry 50 receives the output of the digital pre-distorter30, V_(PD), and generates a predictor that is used by the pre-distorter50 to select the appropriate memory characteristic curve based on theenergy of V_(PD).

Alternatively, as shown in FIG. 5, the predictor circuitry 50 receivesV_(IN) prior to the pre-distorter 30 and makes the prediction based onthe energy of V_(IN). In accordance with the preferred embodiment, thepredictor circuitry 50 is a filter that filters the amplifier input,either prior to or after the pre-distorter 30, and dynamically predictsthe operating temperature of the amplifier 40. A variety of differentfilters can be used as predictor circuitry for this purpose, such as,for example, a moving average (MA) filter, an infinite impulse response(IIR) filter, a lowpass (LP) filter, a finite impulse response (FIR)filter, etc. Those skilled in the art will understand, in view of thedisclosure provided herein, the manner in which these various types offilters can be used to predict the amount of energy dissipated in theamplifier 40.

FIG. 6 is a block diagram of the digital pre-distorter 30 of the presentinvention in accordance with the preferred embodiment. The pre-distorter30 preferably comprises a digital signal processor (DSP) 60, selectioncircuitry 70 and polynomial calculation circuitry 80. The DSP 60 may beany type of processor capable of performing the calculations describedherein. The DSP 60 selects the appropriate memory characteristic outputcurve coefficients based on the predictor generated by the predictorcircuitry 50. The polynomial calculation circuitry 80 applies theselected coefficients to the pre-distorter input signal to produce thepre-distorted output signal.

In order to obtain the output characteristic curves for differenttemperatures, T1 through TN, where N corresponds to the number oftemperatures for which curves are obtained, a training procedure isperformed during which input signals of varying amplitude and phase areinput to the amplifier at different temperatures and measurements aretaken to determine what the temperature output characteristic curve isfor each given amplifier input. Then, during operation, the DSP 60 usesthe predictor output by the predictor circuitry 50 to select thecoefficients to be injected into the input signal. These selections aremade in real-time on the fly by the DSP 60 such that even fast memoryeffects are compensated instantaneously.

This memory compensation architecture represented by FIG. 6 is capableof infinite precision by interpolating the polynomial coefficients ofthe N characteristics curves (e.g., T1-T8) obtained through the trainingprocedure. An efficient recursive computation form of the pre-distorterpolynomial is obtained by the DSP 60 during the training session asfollows: ${{AM}/{AM}},{{{AM}/{PM}} = \begin{matrix}{{k\quad 1*y} + {k\quad 2*y^{2}} + {k\quad 3*y^{3}} + {k\quad 4*y^{4}} + {k\quad 5*y^{5}} + {k\quad 6*y^{6}} + {k\quad 7*y^{7}}} \\{= {y( {{k\quad 1} + {k\quad 2*y^{1}} + {k\quad 3*y^{2}} + {k\quad 4*y^{3}} + {k\quad 5*y^{4}} + {k\quad 6*y^{5}} + {k\quad 7*y^{6}}} )}} \\{= {y( {{k\quad 1} + {y( {{k\quad 2} + {k\quad 3*y^{1}} + {k\quad 4*y^{2}} + {k\quad 5*y^{3}} + {k\quad 6*y^{4}} + {k\quad 7*y^{5}}} )}} )}} \\{= {y( {{k\quad 1} + {y( {{k\quad 2} + {y( {{k\quad 3} + {k\quad 4*y^{1}} + {k\quad 5*y^{2}} + {k\quad 6*y^{3}} + {k\quad 7*y^{4}}} )}} )}} )}} \\{= {y( {{k\quad 1} + {y( {{k\quad 2} + {y( {{k\quad 3} + {y( {{k\quad 4} + {k\quad 5*y^{1}} + {k\quad 6*y^{2}} + {k\quad 7*y^{3}}} )}} )}} )}} )}} \\{= {y( {{k\quad 1} + {y( {{k\quad 2} + {y( {{k\quad 3} + {y( {{k\quad 4} + {y( {{k\quad 5} + {k\quad 6*y^{1}} + {k\quad 7*y^{2}}} )}} )}} )}} )}} )}} \\{= {y( {{k\quad 1} + {y( {{k\quad 2} + {y( {{k\quad 3} + {y( {{k\quad 4} + {y( {{k\quad 5} + {y( {{k\quad 6} + {k\quad 7*y^{1}}} )}} )}} )}} )}} )}} )}}\end{matrix}}$This realization allows the DSP or control logic 60 to perform fewermultiplications than would otherwise have to be performed (e.g., whereN=8, 7 multiplications instead of 35).

As can be seen in FIG. 6, the DSP 60 outputs eight K1 coefficientscorresponding to temperature characteristic curves for temperaturesT1-T8 and multiplexer (MUX) 71 selects the correct K1 coefficient basedon the current temperature predictor output from the predictor circuitry50. Likewise, the DSP 60 outputs eight K2 coefficients corresponding tothe memory characteristic curves for temperatures T1-T8 and MUX 72selects the correct K2 coefficient based on the current temperaturepredictor output from the predictor circuitry 50. The same proceduresare performed with respect to MUXes 73-77 for coefficients K3-K7,respectively.

Once the coefficients have been selected by the MUXes 71-77, they areprovided to the polynomial calculation circuitry 80, which applies thecoefficients to the input of the pre-distorter 30. The polynomialcalculation circuitry 80 has multiplication and addition operators 81and 82, respectively, that apply the coefficients to the input signal inaccordance with the last line of the above interpolation, specifically:Y(k1+y(k2+y(k3+y(k4+y(k5+y(k6+k7*y)))))),where Y corresponds to the input signal being operated upon by thepolynomial calculation circuitry 80 and k1-k7 correspond to thecoefficients.

The present invention provides many benefits. One benefit is that theneed to use expensive ROM lookup tables to store coefficients for manycharacteristic curves is avoided, which reduces the cost, complexity andsize of the pre-distorter 30. Another advantage of the present inventionis that using the temperature predictor of the invention eliminates theneed to use feedback temperature sensors, which typically require ananalog-to-digital converter, a frequency down converter and physicalaccess to the power amplifier transistor. Yet another advantage of thepresent invention is that the fast memory tracking it provides enablesinfinite precision temperature tracking to be performed using a singleMUX selector signal. Yet another advantage of the present invention isthat infinite precision temperature tracking can be achieved through theinterpolation technique of the present invention.

FIG. 7 illustrates a flow chart of the method of the present inventionfor performing pre-distortion in accordance with the preferredembodiment. The first step in the method involves the training session,which is represented by block 101. As stated above, during the trainingsession, different input signals having pre-selected amplitude and phasesweep are input to the amplifier. The amplifier output signals arecaptured and polynomials are fitted to the captured data. This processis repeated for different temperature settings. The DSP or control logic60 records the temperatures that correspond to the different curves. TheDSP or control logic 60 obtains the inverse of each amplifier outputcharacteristic polynomial and stores the coefficients of each inversepolynomial in registers in the DSP 60, or some memory device (notshown).

During operation, the predictor circuitry 50 predicts the temperature ofthe amplifier based on the energy of the input signal, as indicated byblock 102. The appropriate inverse polynomial coefficients for bothphase and magnitude are then selected by the DSP 60 based on the resultsof the prediction, multiplexed by the MUXes 71-77 and applied by thepolynomial calculation circuitry 80 to the input signal to compensatefor amplitude and phase, as indicated by block 103.

It should be noted that the present invention is not limited to theembodiments described herein. For example although reference has beenmade above to the use of the invention with a power amplifier, theinvention applies equally to other types of amplifiers and to othertypes of circuits, indeed, any entity that exhibits nonlinear behaviorin response to a high level input stimuli. Also, the block diagram ofFIG. 6 shows one particular circuit configuration for calculating,selecting and applying the polynomial coefficients. The presentinvention is not limited to the configuration shown in FIG. 6. Forexample, the functions performed by the MUXes 71-77 can be performedwithin the DSP 60. However, using the MUXes 71-77 makes it possible toselect all of the coefficients for a given curve using a single selectorsignal, which is the output of the predictor circuitry 70. The output ofthe predictor circuitry 70 may be conditioned by some hardware orsoftware (not shown) before being input to the MUXes 71-77.Alternatively, the predictor signal output from the predictor circuitry70 may be input to the DSP 60 and processed by software and/or hardwareof the DSP 60. Also, control logic other than a DSP may be used in placeof the DSP. The control logic may be any type of hardware or combinationof hardware and software, or even partially or wholly made up ofdiscrete circuit components. Those skilled in the art will understand,in view of the description provided herein, that other configurationsmay be used to achieve the goals of the invention. In addition, othermodifications may be made to the embodiments described herein and allsuch modifications are within the scope of the invention.

1. A computer program for calculating inverse polynomial coefficientsthat are to be applied to an amplifier input signal to pre-distort theamplifier input signal to compensate for non-linearities of theamplifier, the program being embodied on a computer-readable medium, theprogram comprising: a first routine for capturing a plurality ofamplifier output signals produced by the amplifier in response to theamplifier being stimulated by a plurality of respective amplifier inputsignals, the output signals corresponding to respective amplifiertemperatures; a second routine for fitting respective polynomials torespective data sets representing respective output signals; a thirdroutine for determining respective inverses of the respectivepolynomials; and a fourth routine for obtaining respective sets ofinverse polynomial coefficients from the respective inverses of thepolynomials, the respective sets of coefficients corresponding to saidrespective temperatures.
 2. The computer program of claim 1, furthercomprising: a fifth routine for selecting one of the sets of inversepolynomial coefficients to be applied to an amplifier input signal basedon a predicted temperature of the amplifier.